In the field of graphic arts, printing of a printing plate is carried out using a set of color separation films produced from a color original with the aid of lithographic films. Prior to going into real printing (practical printing operation), color proofs are generally produced from color separation films in order to check up on errors in the step of color separation and necessity to correct colors And it is desired that the color proofs ensure high resolution enabling high-quality reproduction of medium-tone images and high process consistency. In order to obtain color proofs closely analogous to real prints, it is appropriate that materials used for real prints be used as materials for color proofs. Specifically, it is desirable to adopt printing paper used in real printing as a substrate and pigments as coloring materials. Further, a dry process using no developing solution is in great request as a method of producing color proofs.
As a dry process for producing color proofs, the recording system of producing color proofs directly from digital signals has been developed as electronified systems have come into wide use in recent pre-press processes. These electronified systems are utilized with the aim of producing high-quality color proofs in particular, and enable reproduction of halftone images in resolutions of 150 lines/inch. In order to record digital signals in proofs of high quality, laser light capable of being modulated by digital signals and sharply focusing recording light is used as a recording head. Therefore, it becomes necessary to develop image-forming materials with high resolution enabling reproduction of high-definition dots.
As an image-forming material used in a laser light-utilized transfer image formation method, there is known the heat-fusion transfer sheet (Japanese Patent Laid-Open No. 58045/1993) having on a substrate a light-to-heat conversion layer, which can absorb laser light and evolve heat, and an image-forming layer containing pigments dispersed in a medium, such as heat-fusible wax or binder, in order of mention. According to the image-forming method using such a recording material, the light-to-heat conversion layer evolves heat in the laser light-irradiated areas, and the image-forming layer is fused by the heat in the areas corresponding to the irradiated areas and transferred onto an image-receiving sheet superimposed on the transfer sheet, thereby forming transfer images on the image-receiving sheet.
Further, Japanese Patent Laid-Open No. 219052/1994 discloses the thermal transfer sheet comprising a substrate provided sequentially with a light-to-heat conversion layer containing a material enabling photothermal energy conversion, a very thin (0.03 to 0.3 μm) heat-releasable layer and an image-forming layer containing coloring materials. In this thermal transfer sheet, the binding force between the image-forming layer and the light-to-heat conversion layer, which are bound by the mediation of the heat-releasable layer, is reduced by irradiation with laser light to result in formation of high-definition images on an image-receiving sheet superimposed on the thermal transfer sheet. The image formation method using such a thermal transfer sheet takes advantage of the so-called ablation. More specifically, the phenomenon utilized therein is as follows. The heat-releasable layer partly decomposes and vaporizes in the areas irradiated with laser light, and so in the areas corresponding thereto the bonding force between the image-forming layer and the light-to-heat conversion layer becomes weak. As a result, the corresponding areas of the image-forming layer are transferred, onto an image-receiving layer superimposed thereon.
Those image-forming methods have advantages that an actual printing paper to which an image-receiving layer (adhesion layer) is attached can be used as a material for image-receiving sheet and multicolored images can be obtained with ease by transferring images of different colors in succession onto an image-receiving sheet. The image formation method utilizing ablation in particular has an advantage of easy formation of high-definition images, and is useful in producing color proofs (DDCP: Direct Digital color Proofs) or high-definition masking images.
In the progressive context of DTP (DeskTop Publishing) environments, a section of using a CTP (Computer To Plate) system was relieved of an intermediate film-unloading process, and there has been the growing need for proofs produced by the DDCP system as an alternative of galley proofs and analog-mode proofs. Further, large-sized DDCP with high definition, high stability and excellent print-matching performance have been desired in recent years.
The laser thermal transfer method enables printing in high resolution, and various systems thereof are known which include (1) a laser sublimation system, (2) a laser ablation system and (3) a laser fusion system. However, all of these systems have a problem that the shape of recorded dots lacks in sharpness. More specifically, the laser sublimation system (1) uses dyes as coloring materials, and so the degree of similarity to prints is insufficient, the dots formed have blurred outlines since sublimation of coloring materials is utilized therein, and satisfactorily high resolution cannot be achieved. On the other hand, the laser ablation system (2) is satisfactory in similarity to prints since pigments are used as coloring materials but, as in the case of the system (1), the dots formed have blurred outlines and sufficiently high resolution cannot ensure since scatter of coloring materials is caused therein. In addition, the laser fusion system (3) cannot ensure sharp outlines because of fluidity of fused matter.
In the process of DDCP, operations of continuously outputting a number of image sheets and automatically stacking them in a printer are frequently carried out. Although hitherto used materials permit automatic stacking of several sheets, they cause a considerable frequency of troubles, including sticking, waving, curling or/and jutting troubles, when it is required to automatically stack, e.g., 20 image sheets by all-night automatic operation. Therefore, the monitoring by an operator is required, and so the automatic operation is virtually impossible as matters stand.